Last feb I had a power outage as multiple disks where writing to the server. Since then I've had 6095 errors in parity.

Now my question - what is the order in which parity is written?

1: Write data disks then calculate parity?
2: Simultaneous Parity Calc?
3: Calc Parity before data is written.

I'm just wonder what I should trust first, data disks or parity.

Thanks for any help you can give.

You should trust parity!   This is because although Unraid can detect that there is a parity mismatch it cannot tell which disk is causing it so you do not really have a lot of choice.
 

There is always going to be a potential problem when the scenario you describe happens in that data being written at the moment of power loss can be lost.    If in doubt you should run file system checks after such an occurrence on each disk to ensure it’s contents are in a consistent state.

 

such occurrences are also the reason why a UPS is a highly recommended component in any Unraid system so the system can close itself in a tidy manner to minimise the chance of issues arising.

itimpi -

Thank you for the reply. I do have a UPS - but it's too small to spin up the server and 16 drives (even one controller at a time) I had to remove it temporarily - which has become 8 months :(

I actually did what you suggested and trusted parity - the array is currently rebuilding with only 4 hours to go.

1 minute ago, KC said:

itimpi -

Thank you for the reply. I do have a UPS - but it's too small to spin up the server and 16 drives (even one controller at a time) I had to remove it temporarily - which has become 8 months

I actually did what you suggested and trusted parity - the array is currently rebuilding with only 4 hours to go.

The file systems that Unraid uses are of the ‘journaling’ type so should be able to recover to a consistent state if needed.    You are never going to know exactly what was being written at the moment of power loss anyway so the key thing is that already safely written data is intact.

Only 2 hours to go, and it corrected 6095 errors. Now if could just see which files the errors where corrected on. However that's a pretty big ask for Unraid.

Thanks again for the input. I appreciate it.

*** Edit

Kind of makes me glad I did a MD5 on all files on my array - might take a week to figure out which (if any) files are damaged ... but I'll find em.

And a side note - I don't mind thrashing my disks. I have spares.

Edited July 19, 20196 yr by KC